1. Field of the Invention
The invention pertains to the field of membrane release technology. More particularly, the invention pertains to the fabrication of membranes with improved mechanical integrity.
2. Description of Related Art
Thin film resonators (TFRs) are thin film acoustic devices which can resonate in the radio frequency (RF) to microwave range, for example, 0.5 to 5 Gigahertz (GHz), in response to an electrical signal. A typical TFR has a piezoelectric film between a first electrode and a second electrode which apply an electric field to the piezoelectric film. The film is made of a piezoelectric crystalline material, such as zinc oxide, aluminum nitride (AlN) or other piezoelectric crystalline material, which exhibits a piezoelectric effect. The piezoelectric effect occurs when the piezoelectric material expands or contracts in response to an electric field applied across the piezoelectric material, for example by the first and second electrodes, or produces charge or current in response to mechanical stress or strain applied to the piezoelectric material. The mechanical resonance frequency (fr) of the film, is defined for a film of uniform thickness as the acoustic velocity (v) in the film divided by two (2) times the thickness (t) of the film or fr=v/2t.
If an RF source is used to apply an alternating electric field of variable frequency to a piezoelectric film, there will be a pronounced response in the mechanical motion of the piezoelectric film as the frequency of the source varies near the film's mechanical resonance frequency. The fact that the piezoelectric film thus yields different amounts of current at different frequencies makes it a useful element in electrical filters, oscillators, or frequency control circuits.
TFRs can be used at radio frequency because piezoelectric films can be deposited as thin films. For example at higher frequencies, such as 0.5-10 GHz, the piezoelectric film can be between 0.4 and 8 microns in thickness. Piezoelectric resonators needed for these higher frequency applications can be made by using techniques similar to those used to manufacture integrated circuits. The TFR structure can be formed on the substrate, such as a silicon (Si), gallium arsenide (GaAs) or other semiconductor substrate, for monolithic integration purposes, such as integration with active semiconductor devices.
However, because the device's response is acoustic (mechanical) in nature, care must be taken to avoid loss of a signal's energy through mechanical coupling to its support structure, the substrate. Methods for accomplishing the necessary isolation include fabricating the devices on (acoustic) reflecting mirrors or fabricating the devices on membrane support structures. The later technique is the principle topic of the current proposal.
Thin film-resonator-based radio frequency filters are made through thin-film depositions, each followed by one or more processing steps. For 2 GHz applications, a 25,000 Angstroms thick piezoelectric film is deposited over a 2,000 Angstrom thick base electrode, which provides electrical connectivity between resonators. The piezoelectric film typically extends across the edges of this patterned base electrode, but film growth tends to be discontinuous in these regions. If attempts are made to fabricate a freestanding membrane from such a segmented film, the cracks at these discontinuities cause the membrane—and the device—to literally fall apart.
There is a need in the art for methods to improve the mechanical integrity of freestanding membranes. Specifically, a fabrication method which avoids destruction of a membrane due to substrate topology is needed.